How do I unlock this weapon?

Assault

Along with regular attachments such as red dot sights, etc., Assault Rifles can be equipped with an under-barrel frag grenade launcher, smoke grenade launcher and shotgun attachment.

AEK-971 Vintovka

Unlock: Starting Assault weapon

Details: The AEK has an "average" firing rate for assault rifles and has really good stopping power, however it suffers from moderate recoil, even when aiming down the sights. The iron sights are very easy to use, and it is very user friendly.

XM8 Prototype

Unlock: 3,000 Assault Points

Details: The XM8 Prototype is a very balanced assault rifle, with approximately the same power as the AEK-971, and high accuracy. This is a very versatile rifle that can be utilized even at medium-to-long distances with burst fire. The iron sights are very simple and open, so you won't need an red dot for this gun. Once you acquire this rifle, there's really no point to the other rifles other than to try them out. XM8 and M416 are pretty much the same except for really minor differences in reload and firing speed.

F2000 Assault

Unlock: 8,000 Assault Points

Details: The F2000 Assault is a powerful assault rifle with a blazing-fast fire rate. However, it suffers from a great amount of recoil. If used in the right hands, the weapon's tendency to sputter out bullets can be an advantage for burst-firing, while also being deadly up-close.

STG.77 AUG

Unlock: 20,000 Assault Points

Details: The AUG is a slightly more powerful assault rifle with less accuracy and an average rate of fire. If utilized properly in close-quarters-combat, it can drop targets really quickly. As you gain more kills and get Star ranks for the weapon, the accuracy is improved and thus can make the AUG a more ranged and deadly weapon.

AN-94 Abakan

Unlock: 28,000 Assault Points

Details: The AN-94 is a great 2-shot burst fire Assault Rifle that is really powerful and accurate. If you can place your shots without relying on spraying, the Abakan is a great weapon for you to use. However, it is a poor choice for close quarters combat.

M416

Unlock: 35,000 Assault Points

Details: The M416 is an assault rifle for almost all situations. It packs a punch, has great accuracy, on par with the XM8 Prototype and AN-94 and relatively short reload time. It has a weird bullet pattern so burst firing is recommended.

M16A4

Unlock: 47,000 Assault Points

Details: The M16 is a burst firing rifle firing 3-round bursts and is the last unlock for the Assault Class. It uses relatively easy iron sights and is both accurate and the recoil is often kept to a minimum. It's only disadvantage is slightly less damage than the AN-94. The M16 is often poor in Close Quarters, much like the AN-94. Easily the most well-rounded assault rifle, especially at greater distances.